Blowout preventers have long utilized the well pressure being contained to urge the packers into tighter engagement. In some prior structures, the entire ram back has been exposed to well pressure which has resulted in too much closing force. In other prior structures well pressure has been excluded from the back of the ram so that no force from the well pressure is exerted on the ram. Excessive pressure on the packers has caused rapid wear from moving the pipe engaged by the packers at high contained well pressure and severe extrusion damage where the resilient material bridges spaces when there is a high pressure differential across the ram.
The H. Allen U.S. Pat. No. 3,102,709 discloses a two piece ram with a portion of the well pressure force being applied to a plate which is movable a short distance axially with respect to the rest of the ram. This structure reduces the pressure force on the packer when the rams are closed against high pressure and thus reduces wear. However, closing these rams with no pipe extending through the preventer's bore and with full ram piston closing pressure damages the ram front packing by deforming the unsupported portion and excessively straining the pipe engaging area.
The A. L. Burns, Jr. et al U.S. Pat. No. 3,379,255 also allows pressure on the back side of the ram but differs in that the ram piston force is applied directly to the packing. With this structure, the operator by proper control of the piston pressures may be able to control the sealing pressures on the packing face to acceptable limits.
Examples of other prior art blowout preventers which transmit well pressure to the back side of the ram are the H. Allen U.S. Pat. No. 2,387,106, the H. Allen U.S. Pat. No. 2,322,269, the G. E. Nevill et al U.S. Pat. No. 2,318,882 and the K. T. Penick et al U.S. Pat. No. 2,193,110.
The H. Allen et al U.S. Pat. No. 2,278,050 discloses a blowout preventer ram packing having recesses at its back face which may be filled with cork. The stated purpose is ". . . to afford an elasticity to feed the rubber forward in the event of wear and to maintain a seal.". There is also a suggestion to provide the well pressure to the piston chamber but there are no passages provided to conduct well pressure to the recesses.